This invention relates to a method of forming an aluminum-silicon coating composition for protecting ferrous metal substrates from corrosion/erosion, metal dusting, carburization, and other types of high temperature and oxidation interactions which occur during hydrocarbon processing operations.
Various hydrocarbon processing operations including the thermal decomposition of organic compounds, such as the cracking or disproportionation of hydrocarbons, coal gasification etc. have been carried out using steel alloy equipment. While such metal alloys have been particularly useful in increasing the performance life of the respective equipment, problems such as carburization, corrosion and coke deposition are still of concern. One such problem that arises is carburization of the metal which involves diffusion of carbon into the metal which results in embrittlement and can lead to metal loss and eventual failure of the equipment.
A variety of coatings and techniques have been tried to overcome the different problems of the aforesaid types.
Metallic overlay coatings including aluminum and small percentages of silicon have been placed on ferrous metal surfaces to prevent carburization, see British Pat. No. 1,449,260 and U.S. Pat. No. 3,827,967. Metal-ceramic coatings have also been employed, viz., aluminum oxide dispersed in chromium as described in U.S. Pat. No. 3,536,776 but adherence of the preformed oxide to the metal substrate is notably inferior as compared with growing the oxide in situ.
McGill and Weinbaum in Metal Progress, 26, February 1979, have proposed diffusing aluminum vapor into pyrolysis tubes, however, in this method diffusion of aluminum can continue with loss of aluminum into the interior of the tube wall.
Silicon oxide films may be developed on steel surfaces by pretreatment of the bulk alloy containing silicon with steam at elevated temperatures and are said to provide protection against carburization as disclosed in U.S. Pat. No. 3,704,333. Since silicon is a ferrite stabilizer, the amount that can be incorporated in austenitic stainless steels--which generally are used for hydrocarbon pyrolysis operations--is low, of the order of 1 to 2%. In U.S. Pat. No. 4,248,629 the bulk alloy contains silicon and aluminum, both in small amounts.
Duplex or two-layer coatings which require application of two different compositions in sequence has also been disclosed, for example in Arcolin et al., Plasma Spray Conference, The Hague, May 1980, p. 84. In general, they are less practical because of factors of time, more complex operations, unsuitability for application onsite, and the like. See also British Pat. No. 1,529,441 in which three distinct steps may be employed.
Other metal or ceramic coatings have been disclosed to prevent carburization or for other non-specific purposes, see U.S. Pat. No. 3,620,693 and Miller et al., Metal Progress, 103, 80, No. 3 (1973). Vitreous coatings on metals are known as disclosed in U.S. Pat. No. 2,976,171 and No. 4,149,910.
Tien and Pettit, Metallurgical Transactions, 3, 1587 (1972) have shown that yttrium improves the adherence of an Al.sub.2 O.sub.3 scale which develops during oxidation of a Fe-25Cr-4Al alloy.
U.S. Pat. No. 4,190,443 discloses the flame spraying of eutectics, e.g. TiSi.sub.2 plus Si, mixed with another metal power such as Ni, with a final percentage of silicon of 8%. This is said to be an improvement of U.S. Pat. No. 4,039,318 which discloses TiSi.sub.2 with Al and Ni powders. Flame spraying of metal powders requiring the use of a torch is inapplicable to tubes of narrow internal diameter and long length, used in hydrocarbon pyrolysis. Furthermore, such coatings are too porous to be effective at high temperatures involving gaseous species.
The use of fugitive binders to form Al-Si coatings containing up to 10% silicon, is taught in U.S. Pat. No. 3,102,044.
Some of the coatings that have been proposed contain low amounts of silicon. At the other end of the spectrum, coatings of very high silicon content have been produced but only on special metal substrates. Thus, Packer and Perkins in JI, Less Common Metals, 37, 361 (1974), discussed the development of fused slurry silicide coatings for tantalum alloys for use at 1427.degree.-1538.degree. C. Coatings having Si contents in the range of 53-64% were found most effective on tantalum. One problem mentioned by the authors is the volatilization of SiO under conditions of low oxygen partial pressures. This is a condition known to be present in steam cracking, particularly at high temperatures and low steam dilution.
Similarly, Priceman and Sama reported in Electrochemical Technology, 6, 315, No. 9-10, September, October (1968) the use of elemental powders in an organic binder sprayed on a columbium part, then fired, a preferred composition being 60Si-20Cr-20Fe which forms silicides of columbium, chromium and iron. Young and Deadmore describe in Thin Solid Films, 73, 373 (1980) an Al-Si coating formed by spraying an elemental silicon powder slurry on nickel-base superalloy specimens followed by a pack aluminizing treatment at 1100.degree. C. for 16 hours in argon, which is basically aluminizing, viz., a diffusion process. This is a duplex coating process with the inconvenience which that entails. Elbar b.v. Industrieterrain "Spikweien" have described their product, Elcoat 360, as a high silicon content (20 to 25%) coating on In 738, a nickel base alloy, forming a final dispersion of stable silicide phases and suitable for turbine applications.
On the other hand, Fitzer et al., in "Materials and Coatings to Resist High Temperature Corrosion" Edited by D. R. Holmes and A. Rahmel, Applied Science Publishers, Ltd., London, 313 (1980) reported the difficulty of protecting ferrous metals against high temperature oxidation by means of silicon-containing coatings because of high reactivity of silicon towards iron. As a consequence of this, asymmetric interdiffusion of both elements occurs, leading to immediate impairment of the coatings (the Kirkendall effect). In work with nickel base alloys they found it expedient to aluminize prior to slurry coating with CrSi.sub.2 /NiSi.sub.2, thus a duplex coating process. However, the properties of the product were not satisfactory. Further work reported in Thin Solid Films, 64, 305 (1979) on iron base alloys led to duplex coatings with lower Si content, viz., aluminized AISI310 with NiCr15TaSi10 interlayer.
Other literature on coatings includes:
U.S. Pat. No. 3,989,863 PA1 Daimer et al., Abstract Booklet International Conference on Metallic Coatings, San Francisco, CA, Apr. 6-10, 1981 PA1 Wohl et al., ibid PA1 Vargas et al., Thin Solid Films 73, 407 (1980) PA1 Brochure 101, 1977, Sermetel Corp., Limerick, PA.
While the above described coatings and techniques do provide some protection from metal substrates involved in high temperature process applications, there still is the need to obtain a coating composition for ferrous substrates which is of fairly simple constitution and can be applied in a relatively easy manner so as to be applicable to a variety of articles and different process applications.